The capacity and speed of communications systems may be increased by transmitting information in optical form over networks composed of optically transmissive nodes, fibers, waveguides, and the like. High capacity optical communications systems require that many optical signals be frequency or wavelength division multiplexed in the components of an optical network. This requires that there be a way of conveniently producing electromagnetic energy at many different frequencies. An ideal device for producing optical energy useful in an optical communications system is a laser.
Wavelength division multiplexing (WDM) is a technology in which multiple wavelengths share the same optical fiber in order to increase the capacity and configurability of networks. Transmitters having high wavelength stability are crucial to WDM networks. Recently, many WDM transmitters having good wavelength stability have been introduced which use uncontrolled-mode-selection lasers. These lasers have a narrow intracavity filter which sets the lasing wavelength. Under the filter are many cavity modes, one of which is selected for lasing by gain nonlinearities. These lasers are uncontrolled; i.e., no attempt is made to actively place a certain cavity mode under the filter. An uncontrolled mode-selection laser comprises a laser containing an intracavity filter under which the cavity mode alignment is uncontrolled. Because of this lack of control, laser mode hops occur if the cavity modes drift too far with respect to the filter and outside the finite region of stability under the filter. The laser mode hops result in transmission errors.
As the channel spacing of WDM networks decreases, short-cavity lasers, such as distributed feedback (DFB) lasers, typically are wavelength stabilized using external means. To avoid external stabilization, the laser cavity can be lengthened, which reduces the lasing frequency shift per amount of amplifier heating power change (for a given amplifier length) and gain change (regardless of amplifier length). To further improve the wavelength stability, the intracavity filter can be completely placed in passive material. Such a laser is an uncontrolled-mode-selection laser, in which the cavity mode-to-filter alignment control is forsaken. Examples include a multiple-stripe array grating in a cavity (MAGIC) laser, a waveguide grating multifrequency laser, and a fiber grating semiconductor laser. Despite the lack of control, uncontrolled-mode-selection lasers lase in a single longitudinal mode via gain nonlinearities.
Although the art of semiconductor lasers is well developed, there remain some problems inherent in this technology. One particular problem is the instability that accompanies a laser at start-up and during subsequent operation. Therefore, a need exists for a method and structure for a wavelength selectable laser having inherent wavelength and single-mode stability.